Contacts for solar cells formed by directing a laser beam with a particular shape on a metal foil over a dielectric region

ABSTRACT

A method of fabricating a solar cell is disclosed. The method can include forming a dielectric region on a surface of a solar cell structure and forming a metal layer on the dielectric layer. The method can also include configuring a laser beam with a particular shape and directing the laser beam with the particular shape on the metal layer, where the particular shape allows a contact to be formed between the metal layer and the solar cell structure.

TECHNICAL FIELD

Embodiments of the subject matter described herein relate generally tosolar cells. More particularly, embodiments of the subject matter relateto solar cell fabrication processes and structures.

BACKGROUND

Solar cells are well known devices for converting solar radiation toelectrical energy. A solar cell has a front side that faces the sunduring normal operation to collect solar radiation and a backsideopposite the front side. Solar radiation impinging on the solar cellcreates electrical charges that may be harnessed to power an externalelectrical circuit, such as a load. The external electrical circuit mayreceive electrical current from the solar cell by way of metal fingersthat are connected to doped regions of the solar cell.

BRIEF SUMMARY

In an embodiment, a method of fabricating a solar cell is disclosed. Themethod includes forming a dielectric region on a surface of a solar cellstructure and forming a metal layer on the dielectric layer. The methodalso includes configuring a laser beam with a particular shape anddirecting the laser beam with the particular shape on the metal layer,where the particular shape allows a contact to be formed between themetal layer and the solar cell structure. In an embodiment, the laserbeam can be a spatially shaped laser beam or a temporally shaped laserbeam. In an embodiment, the solar cell has a front side configured toface the sun during normal operation and a back side opposite the frontside. In an embodiment, the laser beam can be directed onto the solarcell from the front side or from the back side.

In an embodiment, a solar cell fabricated using the above method isdisclosed.

These and other features of the present disclosure will be readilyapparent to persons of ordinary skill in the art upon reading theentirety of this disclosure, which includes the accompanying drawingsand claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the subject matter may be derived byreferring to the detailed description and claims when considered inconjunction with the following figures, wherein like reference numbersrefer to similar elements throughout the figures.

FIG. 1 is a flow chart representation of an example method forfabricating of a solar cell, according to some embodiments;

FIG. 2 is a cross-section of a metal layer on a solar cell structure;

FIG. 3 is a cross-section of directing a laser beam on a metal layerfrom a back side of a solar cell, according to some embodiments;

FIG. 4 is a cross-section of directing a laser beam on a metal layerfrom a front side of a solar cell, according to some embodiments;

FIG. 5 is a graphical representation of a spatial profile, according tosome embodiments;

FIG. 6 is a graphical representation of a temporal profile, according tothe disclosed techniques; and

FIG. 7 is a cross-section of an example solar cell fabricated accordingto the disclosed techniques.

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature andis not intended to limit the embodiments of the subject matter or theapplication and uses of such embodiments. As used herein, the word“exemplary” means “serving as an example, instance, or illustration.”Any implementation described herein as exemplary is not necessarily tobe construed as preferred or advantageous over other implementations.Furthermore, there is no intention to be bound by any expressed orimplied theory presented in the preceding technical field, background,brief summary or the following detailed description.

This specification includes references to “one embodiment” or “anembodiment.” The appearances of the phrases “in one embodiment” or “inan embodiment” do not necessarily refer to the same embodiment.Particular features, structures, or characteristics may be combined inany suitable manner consistent with this disclosure.

Terminology. The following paragraphs provide definitions and/or contextfor terms found in this disclosure (including the appended claims):

“Comprising.” This term is open-ended. As used in the appended claims,this term does not foreclose additional structure or steps.

“Configured To.” Various units or components may be described or claimedas “configured to” perform a task or tasks. In such contexts,“configured to” is used to connote structure by indicating that theunits/components include structure that performs those task or tasksduring operation. As such, the unit/component can be said to beconfigured to perform the task even when the specified unit/component isnot currently operational (e.g., is not on/active). Reciting that aunit/circuit/component is “configured to” perform one or more tasks isexpressly intended not to invoke 35 U.S.C. §112, sixth paragraph, forthat unit/component.

“First,” “Second,” etc. As used herein, these terms are used as labelsfor nouns that they precede, and do not imply any type of ordering(e.g., spatial, temporal, logical, etc.). For example, reference to a“first” solar cell does not necessarily imply that this solar cell isthe first solar cell in a sequence; instead the term “first” is used todifferentiate this solar cell from another solar cell (e.g., a “second”solar cell).

“Coupled”—The following description refers to elements or nodes orfeatures being “coupled” together. As used herein, unless expresslystated otherwise, “coupled” means that one element/node/feature isdirectly or indirectly joined to (or directly or indirectly communicateswith) another element/node/feature, and not necessarily mechanically.

In addition, certain terminology may also be used in the followingdescription for the purpose of reference only, and thus are not intendedto be limiting. For example, terms such as “upper”, “lower”, “above”,and “below” refer to directions in the drawings to which reference ismade. Terms such as “front”, “back”, “rear”, “side”, “outboard”, and“inboard” describe the orientation and/or location of portions of thecomponent within a consistent but arbitrary frame of reference which ismade clear by reference to the text and the associated drawingsdescribing the component under discussion. Such terminology may includethe words specifically mentioned above, derivatives thereof, and wordsof similar import.

Although much of the disclosure is described in terms of solar cells forease of understanding, the disclosed techniques and structures applyequally to other semiconductor structures (e.g., silicon wafersgenerally).

The formation of metal regions, such as positive and negative busbarsand contact fingers to doped regions on a solar cell can be achallenging process. Techniques and structures disclosed herein improveprecision throughput and cost for related fabrication processes.

In the present disclosure, numerous specific details are provided, suchas examples of structures and methods, to provide a thoroughunderstanding of embodiments. Persons of ordinary skill in the art willrecognize, however, that the embodiments can be practiced without one ormore of the specific details. In other instances, well-known details arenot shown or described to avoid obscuring aspects of the embodiments.

FIG. 1 illustrates a flow chart of an embodiment for an examplefabrication method for a solar cell. In various embodiments, the methodof FIG. 1 can include additional (or fewer) blocks than illustrated. Themethod of FIG. 1 can be performed at the cell level during fabricationof the solar cell or at the module level when the solar cell isconnected and packaged with other solar cells. The example method ofFIG. 1 is first described followed by examples illustrating the stagesof the method at FIGS. 2-4.

As shown in 102, a dielectric region, which can also be referred to as adielectric layer or a passivation layer, can be formed on a surface of asolar cell structure. In an embodiment, the dielectric region can beformed over an N-type doped region and a P-type doped region of thesolar cell structure. In one embodiment, the dielectric region is acontinuous and conformal layer that is formed by blanket deposition. Inan embodiment, the dielectric region can be formed by screen printing,spin coating, or by deposition (Chemical Vapor Deposition CVD,plasma-enhanced chemical vapor deposition (PECVD) or Physical VaporDeposition PVD) and patterning, for example. In various embodiments, thedielectric region can include silicon nitride, silicon oxide, siliconoxynitride, aluminum oxide, amorphous silicon or polysilicon.

In one embodiment, the dielectric region can be partially removed (e.g.,patterned) forming a contact region. In an embodiment, a laser beam canbe directed on the dielectric region to partially remove the dielectricregion. Note that in other embodiments, the dielectric region can beformed in a pattern and not need to be patterned after being formed. Inan embodiment, the dielectric region need not be partially removed.

In an embodiment, the contact region can allow for the formation of acontact, such as an ohmic contact. In some embodiments, the dielectricregion can be maintained between the ohmic contact and the siliconsubstrate (e.g., no dissociation of the dielectric region) whereas inother embodiments, the contact can be in direct contact with the siliconsubstrate, where the dielectric region dissociates. In an embodiment,the dielectric region is partially removed at a particular location,with the particular location being aligned over a N-type doped region ora P-type doped region of the solar cell structure. At 104, a metal layercan be formed on the dielectric region. In one embodiment, the metallayer is a continuous and conformal layer that is formed by blanketdeposition. In an embodiment, forming a metal layer can includeperforming a physical vapor deposition, screen printing, sintering,plating or laser transfer process. In an embodiment, the metal layer canalso be referred to as a seed metal layer. In an embodiment, forming themetal layer can include depositing a seed metal layer on the dielectricregion. In an embodiment, the metal layer can include a metal foil. Inan embodiment, the metal layer can be of at least of a particularthickness to conduct current. In an embodiment, the metal layer can havea thickness in the range of 1-5 microns, for example the metal layer canbe in the range of approximately 1-2 microns (e.g. a seed metal layer).In an embodiment, the metal layer can have a thickness in the range of1-100 microns (e.g. a metal foil), for example the metal layer can beapproximately 50 microns. In an embodiment, the metal layer can includea metal such as, but not limited to, copper, tin, aluminum, silver,gold, chromium, iron, nickel, zinc, ruthenium, palladium, or platinumand their alloys. In an embodiment, the metal layer can be a patternedmetal layer. In an embodiment the patterned metal layer can be placed,deposited or aligned on the dielectric region. In an embodiment,portions of the metal layer can be partially removed to form aninterdigitated pattern.

An example illustration of the fabrication process described at blocks102 and 104 is shown as a cross-section of a solar cell at FIG. 2, asdescribed below.

At 106, a contact can be formed on a solar cell structure. In anembodiment, forming a contact can include configuring a laser beam witha particular shape and directing a laser beam on a metal layer. In anembodiment, directing a laser beam can include directing a locallyconfined energetic beam on the metal layer. In an embodiment, the laserbeam can be spatially or temporally shaped, which can reduce potentialdamage to the solar cell. In an embodiment, the laser used can be a lowpower (e.g., less than 50 milli-Watts) multi-pulse laser. In anembodiment, the laser beam can be generated using a continuous wave (CW)laser or a pulsed laser.

In an embodiment, forming a contact can include forming an ohmiccontact. In an embodiment, the laser beam can be directed on a metalfoil, or other metal layer, to form the ohmic contact on the solar cellstructure. In various embodiments, the laser can be directed fromdifferent locations relative to the solar cell (e.g., from the frontside, from the back side, etc.), as described herein.

Various examples of block 106 (e.g., front-side laser contact formation,back-side laser contact formation, etc.) are illustrated incross-sections of a solar cell being fabricated at FIGS. 3-4, asdescribed below.

In some embodiments, the method of FIG. 1 can be performed for multiplesolar cells at a time. For example, in one embodiment, a metal foil(e.g., including contact fingers for multiple cells) can be aligned andplaced on both a first solar cell and a second solar cell. The metalfoil can then be coupled to both the first and second solar cellaccording to the method of FIG. 1.

FIGS. 2-4 and 7 are cross-sectional views that schematically illustratethe method of FIG. 1.

With reference to FIG. 2, a solar cell during a fabrication process isshown that includes a metal layer 230 placed on a solar cell structure200. As shown, the solar cell structure 200 can include a siliconsubstrate 208, a first doped region 210 or a second doped region 212 anda dielectric region 220. The solar cell of FIG. 2 can also include afront side 204, configured to face the sun during normal operation ofthe solar cell and a back side 202 opposite the front side. As discussedabove, the metal layer can include a metal such as, but not limited to,copper, tin, aluminum, silver, gold, chromium, iron, nickel, zinc,ruthenium, palladium, or platinum, and their alloys. In an embodiment,the dielectric region can include silicon nitride, silicon oxide,silicon oxynitride, aluminum oxide, amorphous silicon or polysilicon. Inan embodiment, the first doped region 210 or the second doped region 212can include a P-type doped region or an N-type doped region of thesilicon substrate 208. In an embodiment, the ohmic contact is alignedwith a particular region of the solar cell structure 200, such asaligned to a P-type doped region or an N-type doped region.

FIGS. 3 and 4 illustrate directing a laser beam 262 from a laser source260 with the particular shape on the metal layer 230 to form a contact240. In an embodiment, the laser beam can be directed on the back side202 of the solar cell as shown in FIG. 3. Provided the laser beam 262 isdirected from the back side 202, a laser beam 262 having a spectrum suchas ultraviolet, infrared and green can be used. In an embodiment, thelaser beam 262 can be directed on the front side 204 of the solar cellas shown in FIG. 4. Provided the laser beam 262 is directed from thefront side 204, the laser beam 262 can have a wavelength greater than 10microns. In an embodiment, the laser beam 262 can be directed from thefront side 204 of the solar cell, where the laser beam 262 can betransmitted through solar cell structure 200 heating the metal layer 230to form the contact 240. In an embodiment, the contact 240 can be formedby a laser welding, laser ablation or laser heating process. In anembodiment, the contact 240 can be formed by configuring the laser beam262 to have either a spatial or temporal profile, where the spatial ortemporal profile can allow for heating of the metal layer 240 anddielectric region 220. In an embodiment, the contact 240 can be formedby configuring the laser beam 262 to form the contact 240 withoutexcessively damaging the irradiated region directed for contactformation region 264. In an embodiment, heating the dielectric region220 with a spatial or temporal profile can dissociate the dielectrics,such as melting amorphous silicon (a-Si) to form a contact 240. In anembodiment, the contact 240 can be an ohmic contact. In an embodiment,the ohmic contact can be formed between the metal layer 230 and thesilicon substrate 208. In an embodiment, the contact 240 canmechanically couple the metal layer 230 to the solar cell structure 200.

With reference to FIGS. 5 and 6, example laser beam intensity profilesare shown. Example spatial profiles can include a top-hat spatialprofile (shown as “A” in FIG. 5), a Gaussian spatial profile (shown as“B” in FIG. 5) and a donut shaped spatial profile (shown as “C” in FIG.5), although other spatial profiles can be used. In an embodiment, thelaser beam can be spatially shaped, such as shown in FIG. 5, where thespatial beam spot profile is configured to form an ohmic contact withoutexcessively damaging the center of the contact formation region 264 fromFIGS. 3 and 4.

Referring to FIG. 6, an example temporal profile is shown. In FIG. 6,the temporal profile shows a first high intensity pulse (shown as “A” inFIG. 6) for dissociating the dielectric region 220 underneath the metallayer 230 and subsequently the followed by a continuous low intensitypulse (shown as “B” in FIG. 6) to form the contact 240 (e.g., anon-abrasive ohmic contact).

FIG. 7 illustrates a solar cell subsequent to the process performed inFIGS. 2-6. The solar cell of FIG. 7 can include a front side 204,configured to face the sun during normal operation of the solar cell anda back side 202 opposite the front side. As shown, the solar cell caninclude a solar cell structure 200. The solar cell 200 structure caninclude a silicon substrate 208, first and second doped regions 210, 212and a dielectric region 220. The solar cell structure 200 is coupled tothe metal layer 230 by a contact 240, such as an ohmic contact. Contactfingers, made up of the first and second metal layers 230, 232 can beseparated. It is to be noted that electrical connection at theseparation can allow for an electrical short and can be detrimental tothe performance of the solar cell. The gap or separation, as shown inFIG. 7, can be formed by a laser ablation process, removing excess metalfrom the metal layer 230. In an embodiment, the first and second dopedregion can be P-type and N-type doped regions. In an embodiment, thedielectric region 220 can be patterned such that some areas do not havedielectric regions under the metal layer 230. In an embodiment, themetal layer 230 can be a metal foil. In an embodiment, the metal foilcan be composed of aluminum. In an embodiment, the metal layer 230 canbe a patterned metal foil. In an embodiment, the patterned metal foilcan be placed on the solar cell structure 200. In an embodiment,portions of the metal layer 230 can be removed in an interdigitatedpattern prior to directing the laser beam. In an embodiment, the metallayer 230 can have a thickness in the range of 1-5 microns, for examplethe metal layer 230 can be in the range of approximately 1-2 microns(e.g. a seed metal layer). In an embodiment, the metal layer 232 canhave a thickness in the range of 1-100 microns (e.g. a metal foil), forexample the metal layer 232 can be approximately 50 microns.

While at least one exemplary embodiment has been presented in theforegoing detailed description, it should be appreciated that a vastnumber of variations exist. It should also be appreciated that theexemplary embodiment or embodiments described herein are not intended tolimit the scope, applicability, or configuration of the claimed subjectmatter in any way. Rather, the foregoing detailed description willprovide those skilled in the art with a convenient road map forimplementing the described embodiment or embodiments. It should beunderstood that various changes can be made in the function andarrangement of elements without departing from the scope defined by theclaims, which includes known equivalents and foreseeable equivalents atthe time of filing this patent application.

What is claimed is:
 1. A method of fabricating a solar cell, the methodcomprising: forming a dielectric region on a surface of a solar cellstructure, the solar cell structure comprising a substrate; forming ametal foil on the dielectric region; configuring a laser beam with aparticular shape; and directing the laser beam with the particular shapeon the metal foil, wherein the particular shape allows a contact to beformed between the metal foil and the solar cell structure, wherein themetal foil stays intact over the contact; and subsequent to forming thecontact between the metal foil and the solar cell structure with thelaser beam, patterning the metal foil using a laser ablation process ina location different from a location of the contact, wherein patterningthe metal foil using the laser ablation process comprises patterning themetal foil without patterning the substrate of the solar cell structure.2. The method of claim 1, wherein configuring a laser beam with aparticular shape comprises spatially shaping the laser beam.
 3. Themethod of claim 2, wherein spatially shaping the laser beam comprisesforming a top-hat spatial profile.
 4. The method of claim 1, whereinconfiguring a laser beam with a particular shape comprises temporallyshaping the laser beam.
 5. The method of claim 4, wherein temporallyshaping the laser beam comprises configuring the laser beam with a lowpower multi-pulse laser process.
 6. The method of claim 1, whereinforming a contact comprises forming an ohmic contact between the metalfoil and the solar cell structure.
 7. The method of claim 1, whereinforming a dielectric region comprises performing a method selected fromthe group consisting of screen printing, spin coating, plating, chemicalvapor deposition process (CVD), plasma-enhanced chemical vapordeposition (PECVD) and physical vapor deposition (PVD).
 8. The method ofclaim 1, wherein forming a metal foil comprises placing a patternedmetal foil on the solar cell structure.
 9. The method of claim 1,wherein forming a metal foil comprises placing a non-patterned metalfoil on the solar cell structure.
 10. The method of claim 1 furthercomprising partially removing portions of the metal foil in aninterdigitated pattern.
 11. The method of claim 1, wherein prior toforming a metal foil, partially removing the dielectric region inpreparation to forming a contact between the metal foil and the solarcell structure.
 12. The method of claim 11, wherein partially removingthe dielectric region comprises directing a laser beam on the dielectricregion.
 13. A method of fabricating a solar cell comprising a solarcell, the solar cell having a front side configured to face the sunduring normal operation and a back side opposite the front side, and themethod comprising: forming a dielectric region on a surface of a solarcell structure, the solar cell structure comprising a substrate; forminga metal foil on the dielectric region; configuring a laser beam with aparticular shape; and directing the laser beam with the particular shapeon the back side of solar cell structure, wherein the particular shapeallows an ohmic contact to be formed between the metal foil and thesolar cell structure, wherein the metal foil stays intact over the ohmiccontact; and subsequent to forming the ohmic contact between the metalfoil and the solar cell structure with the laser beam, patterning themetal foil using a laser ablation process in a location different from alocation of the ohmic contact, wherein patterning the metal foil usingthe laser ablation process comprises patterning the metal foil withoutpatterning the substrate of the solar cell structure.
 14. The method ofclaim 13, wherein configuring a laser beam with a particular shapecomprises spatially shaping the laser beam.
 15. The method of claim 13,wherein configuring a laser beam comprises forming a top-hat spatialprofile.
 16. The method of claim 13, wherein directing a laser beamcomprises directing a laser beam having a spectrum selected from thegroup consisting of ultraviolet, infrared and green.